Variators for use in motor vehicle transmissions are widely known. The present invention is particularly applicable to variators of the type sometimes referred to as “torque controlled”. The principle upon which they operate is known e.g. from Torotrak's earlier patents (including U.S. Pat. No. 5,395,292 and its European counterpart EP444086) but can briefly be summarised as follows. Whereas more conventional “ratio controlled” variators receive a control input which corresponds to a chosen variator drive ratio (determined by an associated electronic control) and are constructed such as to automatically adjust to provide the chosen ratio, a torque controlled variator instead receives a control input corresponding to chosen torques at the variator input/output. Looking at the specific example of the toroidal-race, rolling-traction type variator described in the aforementioned Torotrak patents, the variator's control input directly determines the “reaction torque” which is the sum of the variator's input and output torques. The actual variator ratio is not directly determined by the control input to the variator. Instead it results from the acceleration/deceleration of the engine and vehicle resulting from the torques respectively exerted thereupon by the variator input and output shafts.
The control input to this type of variator conventionally takes the form of a difference between two hydraulic pressures. By acting on opposed piston faces the two pressures create an adjustable force upon components of the variator (typically in the form of rollers) which in turn serve to create the reaction torque. A pair of hydraulic valves is used to control the two hydraulic pressures. In the associated electronic controller, a requirement for variator reaction torque is converted into a requirement for the two pressures and so into control signals passed to the two valves. Hence the electronic controller has direct control over variator reaction torque.
While the variator ratio is constant, and no flow of fluid takes place between the hydraulic valves and the variator pistons, the hydraulic pressures acting upon the variator pistons are equal to the pressures output from the respective hydraulic valves. However the inventors have recognised that when variator ratio changes, with consequent flow in the hydraulics connecting the valves to the pistons, a pressure drop in the lines along the direction of flow inevitably results. The pressures exerted upon the variator pistons are therefore not the same as the pressures output from the hydraulic valves. The result can be a deviation of the reaction torque from the required value.
The problem is exacerbated because the hydraulics typically incorporate some form of hydraulic damper intended to create a pressure drop in response to fluid flow, thereby to reduce unwanted oscillatory behaviour of the variator itself. The damper contributes to the pressure drop. Through flow of fluid can also prevent the valves from creating the demanded pressures.